The present invention relates to novel radiation-sensitive esters composed of a) a compound containing 2 to 6 aromatic hydroxyl groups, b) a ring-substituted (o-naphthoquinone 2-diazide)-4-sulfonic acid and c) an (o-naphthoquinone 2-diazide)-4- or -5-sulfonic acid which is not further substituted and/or a non-radiation-sensitive organic acid, and also to a process for their preparation.
The radiation-sensitive layer of standard copying materials is essentially composed of a mixture of an alkali-soluble cresol formaldehyde novolak with radiation-sensitive compounds, such as 1,2-benzo- or o-naphthoquinone 2-diazide derivatives. The novolak is soluble per se in aqueous alkaline solutions, but the radiation-sensitive o-quinone diazide compounds act as solution inhibitors. In the imagewise exposure of the layer to actinic radiation, the radiation-sensitive diazocarbonyl compound is converted into a carboxylic acid via a ketene intermediate. The carboxylic acid is readily soluble in aqueous alkaline solution and, consequently, also enhances the solubility of the novolak. The exposed regions of the copying layer dissolve in the alkaline developer solution, while the unexposed regions remain essentially unaltered and intact, with the result that a positive relief image of the master is produced on the layer support.
The solubility behavior described can, however, also be reversed. For this purpose, the recording layer is subjected to a heat treatment after imagewise irradiation. Under these circumstances, the resin molecules of the layer cross-link in the regions of the layer affected by the light. This process, referred to as "hardening" requires, as a rule, the presence of a "cross-linking agent" which brings about the cross-linking and, consequently, the hardening during the heat treatment, under the influence of the acid which has been produced from the o-quinone diazide during the exposure. During hardening heating is carried out to temperatures below the decomposition temperature of the o-quinone diazide. The heating can be carried out by irradiation, placement into a stream of hot gas, contact with heated surfaces, for example with heated rollers, or immersion in a heated bath of an inert liquid, for example water. The temperature is in general between 90.degree. and 150.degree. C., preferably between 100.degree. and 130.degree. C.
Efficient cross-linking agents are generally compounds which readily form a carbonium ion under the acid and temperature conditions described. Examples of these are the hexamethylolmelamine ethers in accordance with DE-A 33 25 022 (=U.S. Pat. No. 4,581,321) and also the compounds described in EP-A 0 212 482, such as 1,4-bishydroxymethylbenzene and 4,4'-bismethoxymethyl diphenyl ether, which contain two or more aromatically bound hydroxymethyl or alkoxymethyl groups. 2,6-Bishydroxymethyl-4-methylphenol in accordance with U.S. Pat. No. 4,404,272 is also known as a cross-linking agent.
After the heat treatment, the photoresist layer is, as a rule, subjected to a whole-surface exposure ("flood exposure") in order to render the still radiation-sensitive layer regions completely alkali-soluble. The flood exposure is in general carried out with the same light source which was also used for the image exposure.
The development following the flood exposure is in general carried out with one of the aqueous alkaline solutions which are also used to develop a positive-working photoresist. These are, for example, aqueous solutions of sodium hydroxide, tetramethylammonium hydroxide, trimethyl(hydroxyethyl)ammonium hydroxide, alkali-metal phosphates, alkali-metal silicates or alkali-metal carbonates, which solutions may contain wetting agents or fairly small amounts of organic solvents. During the development, the layer regions not affected by light in the original image exposure are washed out, with the result that a negative resist image of the master is obtained.
In most cases, the exposed and developed resist material is then treated with an etchant, in which process the etchant is only able to act on the layer support in the non-image regions. In this way, a negative etching image is produced on the layer support in the case of a positive-working copying layer and a positive etching image in the case of a negative-working copying layer.
The positive or negative relief image of the copying layer produced on the layer support by the processes described is suitable for various application purposes, inter alia as an exposure mask or as an image in the production of semiconductor components in microelectronics, as printing forms for letterpress, gravure or lithographic printing, and also for the production of nickel rotation cylinders in an electroplating process.
The commercial suitability of a copying layer, for example a photoresist layer, is assessed, inter alia, on the basis of the radiation sensitivity, the development and image contrast, the resolution and the adhesion to the layer support.
A high radiation sensitivity of the composition is an important factor in the manufacture of microelectronic circuits or components, especially in the so-called "in-line" processing of wafers, in which the throughput of wafers is determined by the longest lasting process step. With the relatively long exposure times hitherto necessary, the exposure equipment throughput is the limiting factor. The exposure equipment cycle times are too long, especially with monochromatic irradiation and with irradiation using shorter-wave actinic light, and this results in an unduly low production rate.
The resist resolution relates to the capability of a photoresist system to reproduce even the finest lines and gaps of a mask used for the exposure, the exposed and developed regions being required to exhibit a high degree of edge steepness and sharpness.
In many technical application fields, in particular in the production of semiconductor components in microelectronics, the photoresist has to achieve a particularly high resolution as very small line and gap widths (&lt;1 .mu.m) have to be reproduced. The ability to reproduce smallest details in the order of magnitude of 1 .mu.m and less is of the greatest importance for the large-scale production of integrated circuits on silicon chips and similar components. If photographic processes are used, the integration density on such a chip can be increased by increasing the resolving power of the photoresist.
It is known that the resolution of a photoresist increases if photoactive compounds containing a plurality of radiation-sensitive radicals in one molecule are present in it since the radiation-sensitive component in the mixture is then increased. [P. Trefonas III and B. K. Daniels, "New Principle for Image Enhancement in Single Layer Positive Photoresists", SPIE Advances in Resist Technology and Processing IV, 771 (1987), 194-210; P. Trefonas III, B. K. Daniels and R. L. Fischer, "Photoresist Design for Submicron Optical Lithography: Application of Polyphotolysis"Solid State Technology 30 (1987), 131-137].
If the mixed esters mentioned in EP-A 0 244 762 and 0 244 763 are used, an improved resolution can already be observed. DE-A 38 37 500 reveals the superior properties of ring-substituted naphthoquinone diazide derivatives in lithographic applications. Esters of optionally substituted (o-naphthoquinone 2-diazide)-4- or -5-sulfonic acids with compounds which possess three or more aromatic hydroxyl groups often have, however, an unduly low solubility in the standard solvents, and this has the result that the resists prepared with these esters do not yet achieve adequate resolution.